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<h4 class="subsection" id="If-1"><span>4.2.2 If<a class="copiable-link" href="#If-1"> &para;</a></span></h4>

<p>The &lsquo;<samp class="samp">#if</samp>&rsquo; directive allows you to test the value of an arithmetic
expression, rather than the mere existence of one macro.  Its syntax is
</p>
<div class="example smallexample">
<div class="group"><pre class="example-preformatted">#if <var class="var">expression</var>

<var class="var">controlled text</var>

#endif /* <var class="var">expression</var> */
</pre></div></div>

<p><var class="var">expression</var> is a C expression of integer type, subject to stringent
restrictions.  It may contain
</p>
<ul class="itemize mark-bullet">
<li>Integer constants.

</li><li>Character constants, which are interpreted as they would be in normal
code.

</li><li>Arithmetic operators for addition, subtraction, multiplication,
division, bitwise operations, shifts, comparisons, and logical
operations (<code class="code">&amp;&amp;</code> and <code class="code">||</code>).  The latter two obey the usual
short-circuiting rules of standard C.

</li><li>Macros.  All macros in the expression are expanded before actual
computation of the expression&rsquo;s value begins.

</li><li>Uses of the <code class="code">defined</code> operator, which lets you check whether macros
are defined in the middle of an &lsquo;<samp class="samp">#if</samp>&rsquo;.

</li><li>Identifiers that are not macros, which are all considered to be the
number zero.  This allows you to write <code class="code">#if&nbsp;MACRO<!-- /@w --></code> instead of
<code class="code">#ifdef&nbsp;MACRO<!-- /@w --></code>, if you know that MACRO, when defined, will
always have a nonzero value.  Function-like macros used without their
function call parentheses are also treated as zero.

<p>In some contexts this shortcut is undesirable.  The <samp class="option">-Wundef</samp>
option causes GCC to warn whenever it encounters an identifier which is
not a macro in an &lsquo;<samp class="samp">#if</samp>&rsquo;.
</p></li></ul>

<p>The preprocessor does not know anything about types in the language.
Therefore, <code class="code">sizeof</code> operators are not recognized in &lsquo;<samp class="samp">#if</samp>&rsquo;, and
neither are <code class="code">enum</code> constants.  They will be taken as identifiers
which are not macros, and replaced by zero.  In the case of
<code class="code">sizeof</code>, this is likely to cause the expression to be invalid.
</p>
<p>The preprocessor calculates the value of <var class="var">expression</var>.  It carries
out all calculations in the widest integer type known to the compiler;
on most machines supported by GCC this is 64 bits.  This is not the same
rule as the compiler uses to calculate the value of a constant
expression, and may give different results in some cases.  If the value
comes out to be nonzero, the &lsquo;<samp class="samp">#if</samp>&rsquo; succeeds and the <var class="var">controlled
text</var> is included; otherwise it is skipped.
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